This invention relates to methods and compositions for gravel packing wells. More particularly, the invention relates to improved methods and compositions for consolidating gravel packs within wash-out cavities surrounding a wellbore.
Sand consolidation and gravel packing are two near wellbore techniques widely used for controlling the production of sand from producing wells such as oil wells, gas wells and similar boreholes. In many instances, highly porous and fragmentable sand formations surround a wellbore. Under production conditions, the sand is often displaced from its aggregated structure and carried along by the fluid flood operations to a producing well. If the sand flow is allowed to proceed unchecked, the producing wellbore soon becomes full of sand, thereby clogging the wellbore and impeding oil production. Furthermore, sand arriving at the surface site of the well erodes the production hardware.
As more and more sand is displaced from its original formation, a region of washed-out cavities surrounding the wellbore region results. As the washed-out zones become more extensive, the integrity of the wellbore is threatened and a danger of the wellbore collapsing exists.
It has therefore been the subject of extensive and intense research by the petroleum industry to develop techniques to minimize or obviate displacement of sand particles into producing well areas and prevent the formation of wash-out cavities surrounding the wellbore. One such general approach suggested by the art is to consolidate the coarse sand structures prior to fluid production. Sand consolidation techniques are aimed at cementing loose sand structures adjacent a wellbore. Such consolidation is effective to prevent breakdown of sand formations and subsequent clogging of producing wells.
However, there are many instances where substantial wash-out cavities are either initially present naturally near the wellbore or, which more frequently occurs, washed-out cavities form around the borehole after prolonged use despite attempts at sand consolidation.
When the problem of sand breakdown has progressed to a point where there are substantial wash-out cavities adjacent the wellbore, resort is made to gravel packing techniques to prevent further erosion and to reestablish the integrity of the wellbore periphery. Gravel packing is a secondary sand consolidation technique involving the introduction of a fluid suspension of exogenous particulate matter downhole, to fill the wash-out cavities. The term gravel is somewhat loosely applied in the art to encompass hard, rigid particulate matter ranging in size from a coarse sand to pebble size material.
Once the replacement of sand and gravel has been accomplished in the wash-out zones, something more must be done to conserve the loose filling material and retard the upstream sand flow through the filler material during production conditions. Generally, gravel packing is followed by the introduction of consolidating material to aggregate filler material such that a hardened permanent structure is obtained which serves as a barrier to the continuous erosion of indigenous sand.
One method that has been used for consolidating gravel packing involves the use of a polymerizable resin to bond the filler material together after initial placement. In this technique, a slurry of liquid resinous material and particulate filler material is pumped into the well. After placement, the thermosetting resin bonds the filler material together to a permeable, consolidated structure. Although this technique is technically feasible, it is quite expensive in its requirement for excess resinous material. Moreover, this technique requires special mixing equipment to ensure a homogenized dispersement of the particulate matter and prevent undesired sedimentation of the particulate matter from the resinous material upon introduction of the slurry down through the wellbore. Furthermore, controlled placement procedures are often required to guard against premature curing of the resin.
A variation of this technique involves a two step operation wherein the particulate matter is initially placed into the wash-out cavities and then a liquid resinous material is injected into the packed cavity to effect consolidation. Unfortunately, the problems associated with this variation is that the resin may fail to uniformly invade the packed cavity and coat all of the particulate matter or, as most often is the case, the polymerization reaction effecting consolidation is difficult to control. Consequently, the polymerization reaction may go unchecked, plugging pore spaces and ultimately blocking permeability through the packed material.
Another approach for consolidating gravel packs is to introduce particles coated with a one-step solid-fusible resin into the wash-out cavity of the subterranean formation. Formation conditions cause the resin to melt and fuse adjacent particles together, thereby creating a consolidated structure. A problem associated with this technique is that the consolidated structure decomposes when subjected to high temperatures such as that encountered during steam stripping procedures wherein the temperature within the formation may be as high as 600.degree. F. At this temperature the fused resin begins to depolymerize, resulting in break down of the aggregated structure. Furthermore, there have been problems associated with placing the sand downhole. One cannot simly drop the sand downhole, rather it must be floated through the borehole to prevent rapid sedimentation of the sand and formation of voids. Accordingly, to prevent this rapid sand settling it is necessary to use a viscous fluid fill. Traditionally, polymers in relatively high concentrations have been incorporated in the fluid suspension of particulate matter to provide sufficient viscosity to suspend the sand polymer thickeners.
It is therefore desirable to provide improved compositions and methods for gravel consolidation which are cost effective, provide for controlled settlement, and are resistant to high temperatures after consolidation has been effected.
The present invention is directed to satisfying the aforementioned objectives. The invention is directed toward providing methods and compositions which are effective to prevent premature settling of particulate matter during well infusion and to provide a heat stable polymerized consolidate.